Managing Permissions in Amazon S3

Access Control Methods Overview

Bucket Policies vs. IAM

Bucket Policies

• Resource-based policies defined at the bucket level.

• Can explicitly allow or deny access to specific principals.

• Takes precedence over other permissions.

• Applied to all objects or a subset within the bucket.

• Defined directly on the bucket resource.

IAM (Identity and Access Management)

• Identity-based policies.

• Grants temporary permissions through roles.

• Can be assigned to users, services, or applications.

• Manageable through AWS Console or SDK.

• Access keys automatically rotate for enhanced security.

Permission Evaluation Process

Single Account Access

1. Bucket Policy Check:

   • Explicit deny: Access blocked.

   • Explicit allow: Access granted regardless of IAM permissions.

   • Neither: Proceeds to IAM check.

2. IAM Permission Check:

   • Evaluates user's IAM policies.

   • Checks group memberships.

   • Verifies role permissions.

Cross-Account Access

1. Source Account Requirements:

   • Create role with appropriate permissions.

   • Establish trust policy allowing external account access.

2. Destination Account Requirements:

   • Create role to assume the source account's role.

   • Grant necessary permissions to principals.

3. Permission Flow:

   • Principal assumes local role.

   • Local role assumes cross-account role.

   • Access granted based on cross-account role permissions.

Networking Considerations

Access Methods

1. Public Internet Access:

   • Via Internet Gateway.

   • Higher data transfer costs.

   • Standard internet latency.

2. S3 Gateway Endpoint:

   • Uses AWS private network.

   • Lower data transfer costs.

   • Reduced latency.

   • Requires VPC route table configuration.

   • Uses Elastic Network Interface.

Best Practices and Key Considerations

Security

• Prefer IAM roles over bucket policies for cross-account access.

• Take advantage of automatic key rotation with IAM roles.

• Use explicit denies in bucket policies for critical restrictions.

Networking

• Implement S3 gateway endpoints for cost-effective internal access.

• Configure appropriate route tables for VPC connectivity.

• Consider data transfer costs when choosing access methods.

Access Management

• Clearly define trust relationships for cross-account access.

• Use temporary credentials through IAM roles when possible.

• Maintain principle of least privilege.

• Regular audit of access patterns and permissions.

Implementation Examples

Principal Types That Can Assume Roles

• IAM Users.

• EC2 Instances.

• Lambda Functions.

• Custom Applications.

Network Configuration

VPC --> S3 Gateway Endpoint --> S3 Bucket
         (Elastic Network Interface)

This configuration provides:

• Private network access.

• Reduced costs.

• Improved latency.

• Secure communication path.